From what I understand, X-Plane attempts to dynamically calculate the behavior of an aircraft in flight, whereas Flight Simulator depends more on tables of data for each aircraft. The result is that Flight Simulator works well for normal flight regimes and real-world aircraft, whereas X-Plane works well for unusual flight regimes and experimental or hypothetical aircraft.
And from what I understand, high-level simulators designed to simulate specific real-world aircraft (such as airliners) are mostly table-driven, because the behavior of the simulator must match the real aircraft, and it's easier to do this by looking up the required behavior in a table of test data obtained from the real aircraft than it is to try to simulate it by applying laws of physics and aerodynamics. The table-driven result tends to be more faithful to the real thing. The drawback is that the sim isn't very good at simulating things for which there is no test data. This would explain recent concerns about high-level simulators not being able to correctly simulate unusual situations, such as spins or inverted flight or engines falling off the wing or whatever. Since there's no test data for these scenarios, the sim has to guess, and it doesn't always guess right.
This in turn means that if you like to simulate sedate, normal flight in aircraft that exist in the real world, MSFS is a good choice. But if you like to experiment with your own aircraft (or any aircraft for which no body of test data exists), or if you like aerobatics or other unusual flight regimes, you might be better off with X-Plane.
Also, I can't agree that you need a fancy video card. With most games these days, a fast video card makes a huge difference in frame rates and performance, but not with Flight Simulator. Flight Simulator depends heavily on the CPU for graphics performance, in part because sometimes it has to calculate each pixel individually. It can't benefit from splashing a texture on polygons for the most part, although it does that a little bit when it can. Polygon performance in a GPU doesn't help for pixel-sized details on the horizon, though.
When I've upgraded video cards, sometimes most games speed up by an order of magnitude, but I hardly see any change in Flight Simulator at all. A faster CPU, on the other hand, will have a noticeable effect on MSFS.
Hmm ... also, XP is a better platform for MSFS than Windows 7. And 32-bit is preferable, since MSFS is a 32-bit app.